The present invention relates to chairs, particularly to office chairs of the type which recline. More specifically, the field of the invention is that of knee tilt control mechanisms for office chairs.
A problem with prior art tilting chairs involves the position of the chair occupant's feet. When a chair seat is tilted about an axis located in the middle of the seat, the rear of the seat is lowered simultaneously with the elevation of the front. The occupant's knee is normally located adjacent to the front of the seat, so that when the front rises so do the occupant's legs thereby causing the feet to be lifted above the ground. This lifting of the legs above the floor is undesirable and many prior art chair structures exist which attempt to solve this problem.
In one prior art chair design, a tilting axis is provided near the front of the seat so that when the chair is tilted, the front edge of the seat remains at relatively the same height above the ground. With such an arrangement, the occupant's legs are not lifted above the ground because the tilting occurs about an axis in the vicinity of the knees. However, prior art knee tilt control mechanisms have problems associated with the amount of resistance against tilting. For example, a high resistance spring causes tilting, particularly in positions of near full tilt, to be unnecessarily difficult as the linear increase of resistance may be too strong for many persons. Further, a low resistance spring causes unwanted tilting because the initial force of the occupant's body sitting down biases the seat in a substantial backward tilt, which is an undesirable position when working at a desk or table.
To overcome the above mentioned problems, one prior art chair includes a mechanical lock for maintaining an upright position. To recline in this chair, the occupant must first unlock the mechanism before reclining. Similarly to return to the upright position, the occupant must move to and then lock the chair in the upright position. The mechanical lock detracts from the comfort and flexibility of the chair because the chair occupant must operate the lock to adjust the position of the chair.
Another prior art chair includes a passive knee tilt mechanism, i.e., a mechanism which responds to the positioning of the occupant. The passive knee tilt mechanism provides a high initial resistance to tilting and then a generally increasing resistance for the remainder of the possible tilting angle. Because of the high initial resistance, the occupant will be in the upright position unless the occupant exerts a backward force. Further, with the exertion of a reasonable amount of force the occupant can easily recline. The knee tilt mechanism includes two springs, one providing a linear resistance to tilting and the other attached to a cam lever so that at a predetermined angle the compression of the other spring remains relatively constant.
Having the tilt axis positioned in front of the spindle axis of a chair allows the seat to recline and potentially impact the spindle hub. The resistance of the springs limit the tilting of the chair. However, a significantly large force can overcome the resistance of the springs, for example a relatively large person falling backwards into the seat.
Another chair which is disclosed in U.S. Pat. No. 4,858,993 has a passive knee tilt mechanism including a spring and a toggle lever. The spring provides a linearly increasing resistance to tilting, and the toggle lever provides a non-linear resistance which at a predetermined angle initially increases sharply then gradually decreases from the maximum level. The spring is adjusted by rotation of a knob connected to a flexible shaft which is threadedly connected to an upper portion of the spring support, the upper portion engaging and moving on a cam guide of the seat. The toggle lever also includes a means for adjusting the angle at which the toggle lever begins to provide resistance so that the chair rests at that angle. However, movement of the spring support in the cam guide must overcome the frictional resistance of the cam surface. Also, the flexible shaft may be less reliable than a directly connected adjusting screw for adjusting the spring.
One feature common to most office chairs having springs involves adjusting the chairs, resistance to tilting. Prior art spring adjustment mechanisms generally use either direct acting or sliding wedge types of adjustment. One type of chair structure uses a direct acting adjustment mechanism wherein an adjustment knob mechanism acts along the centerlines of springs to make the adjustments. Turning the knob is difficult because the spring and friction forces directly resist the axial movement of the knob. Another direct acting adjustment involves double torsion springs, wherein a knob mechanism acts on one of the arms of the spring. Turning the knob is slightly less difficult than with a single spring because some leverage is provided by the structure of the double torsion spring; however, the mechanical advantage gained by double torsion spring structure provides only a marginal improvement, below a 2:1 leverage. Prior art sliding wedge arrangements include a wedge driven by an adjusting screw to compress the springs. Turning the knob is facilitated by the leverage of the wedge and resisted by the friction between wedge surfaces, thus creating a net effect which differs little from direct acting adjustment arrangements.
What is needed is a tiltable chair which is easier to adjust and has an adjustment mechanism which utilizes a greater amount of leverage. Also, a tiltable chair having a more effective positive stop is needed.